A liquid-phase quartz crystal microbalance for photovoltaics research

Processes involving solid surfaces and liquid reactants are ubiquitous in the semiconductor industry, and photovoltaics is no exception. Examples include the chemical bath deposition of CdS buffer layers in Cu(In,Ga)Se2 and CuInS2 devices, a process that is used in virtually every commercially available CIS-based module. Aqueous etching is used for anti-reflective texturizing of silicon cells, and is also used in the cleaning, defect etching, and native oxide removal for that particular PV material. Research on next generation quantum dot-based solar cells frequently uses simple solution-based methods for the growth and modification of the quantum dots. In all of these examples, a liquid is used to remove or deposit very thin, sometimes atomically thin, layers on the surface of a solid, and the nature of the chemical and physical changes that take place at that interface are critical to the proper functioning of the finished photovoltaic device. Despite the widespread use of liquid processes in research and in industry, there are few characterization techniques suitable for in-situ and real-time investigations of the solid-liquid interface. We have recently finished constructing a new, low-cost tool based on a quartz crystal microbalance for investigations of liquid processes that are of interest to the PV community. The tool consists of a liquid-phase QCM, pumps, valving, reaction vessels, membrane contactors for control of dissolved gases, and a PC-based control system, and can be used in continuous flow cell mode or in a batch reaction scheme. Because the system has been integrated with the NREL Surface Analysis Team´s cluster tool, films either grown or modified with this new tool can be transferred without fear of atmospheric contamination to a suite of surface analytical techniques for further study. This paper will describe the new tool, its capabilities, potential pitfalls with its use, initial results, and possible future research dir- - ections.