Dynamics of chemical bath deposition of inorganic thin films for photovoltaic applications

Abstract

Chemical bath deposition (CBD) is a simple, low cost and scalable technique to deposit inorganic films, but lack of fundamental understanding and control of the underlying chemistry has limited its versatility. We combined the use of a continuous-flow microreactor (CFµR), speciation modeling, and novel time-resolved bath characterization techniques to better understand the evolution of the baths and its impact on film formation and properties. The model system for this study was ZnS, a wide band gap semiconductor whose many applications, such as photovoltaic buffer layers, require uniform and continuous films down to several nanometers thick. In CBD of ZnS, oxygen is often incorporated into the film as oxide and hydroxide to form Zn(S,O,OH). Oxide tunes electronic band levels while hydroxide detrimentally affects electronic device stability. Efforts to understand the film formation and gradation of the film stoichiometry and properties are limited by film thicknesses of ~50 nm that are smaller than the probe size of many characterization techniques. We used a CFµR to investigate relationships between bath composition and film properties deposited from a high-performance alkaline recipe. For decreased film oxygen content, the near-neutral deposition regime was explored. Hexamethylenetetramine (HMTA) was successfully used as an additive to maintain near-neutral pH for rapid deposition of continuous film with reduced oxygen impurities. Complexing agents, or ligands, limit the free metal ion concentration and control the film deposition rate, but their dynamic properties are poorly understood. We demonstrated that dynamic speciation modeling is a powerful tool in predicting the ligands’ throttling behavior. Time-dependent deposition rate was captured through spatially-dependent film thickness by CFµR, and confirmed the prediction of the model. Understanding bath dynamics is crucial to advancing CBD from the traditional “recipe-based” perception to a level in which film properties can be fully controlled by bath engineering.%%%%Ph.D., Chemical engineering – Drexel University, 2015