碳化硅太阳能电池的湿式化学处理

ABSTRACT: Wet chemical processes are widely used within crystalline silicon solar cell production, mainly for  surface texturing and cleaning purposes. Whereas research has been focusing mainly on process development in the  past, only little progress could be observed in terms of process control techniques. Within the paper current state-ofthe-art wet chemical production processes are discussed and process control and quality assurance procedures  (chemical, optical and electrical characterization of the wafers) are proposed. Purpose of the advanced  characterization techniques is an enhancement of the process quality as well as an increase in high quality production  yield. Further on, future industrial high efficiency cell processing necessitates cost effective, high quality cleaning  processes especially prior to any surface passivation step. Starting from the well known semiconductor cleaning  sequences, the paper motivates process simplifications and adaptations suitable for PV mass production.  Keywords: wet chemistry, process control, characterization.

Quality assurance and process control for wetchemical applications are becoming increasingly  important in the industrial production chain to  manufacture silicon solar cells. To overcome process  operations based on operator experience as well as to  extend overall operating times of common etching  bathes, new developments for online characterisation and  control will be mandatory.

For the process development, new requirements  emerge from transferring high efficiency cell processes  from lab scale towards industrial production equipment.  Especially higher standards for the wafer and surface  cleaning at various stages within the cell process are  required; nonetheless, production costs and therefore  process complexity have to be kept as low as possible.

Fig1

Additionally, from a scientific point of view, a better  understanding of the underlying mechanisms especially  for the texturing processes could be achieved, which in  the end again will support further process optimization.  As an example for the importance of an elaborate process  control, chemical consumption data for a typical acidic  texturing process can be taken: Assuming an average  industrial bath operating time of around 80h, the amount  of dosed HF and HNO3 during that time period  accumulates to a factor of 10 to 15 higher than the  original amount of HF and HNO3 used for the fresh bath  make-up. These figures demonstrate, that basically the  overall process performance is mainly driven by an  accurate dosing of the consumed chemicals and therefore  their exact determination is of great importance.

In comparison, texture 3 shows lowest reflectances,  but also low shunt resistances and hence low Voc. This  can be attributed to an extremely rough surface structures  with a lot of deep etch pits formed at crystal defects and  grain boundaries. Subsequent SiNx deposition fails to  homogeneously cover these deeply etched areas resulting  in inhomogeneous contact formation and hence low shunt  resistances and conversion efficiencies.