Why is the Open-Circuit Voltage of Crystalline Si Solar Cells so Critically Dependent on Emitter- and Base-Doping?

This paper discusses the critical dependence of the open-circuit voltage (VOC) of crystalline Si solar cells on the emitter and base doping levels. Contrary to conventional models that try to ascribe VOC-limitations to (independent) bulk and surface recombination losses, we suggest, as the dominant mechanism, the formation of a compensated 'buffer layer' that is formed as phosphorus is diffusedinto the p-type (boron-doped) base. The only purpose of the base doping is to optimize the buffer layer. Our calculations show that this model makes the achievement of high VOC and good carrier collection (JSC, FF) interdependent. Sanyo's 'HIT' solar cells are an example of a different method to implement this buffer layer concept for crystalline Si solar cells. The general principle for aVOC-enhancing buffer layer relies on using materials with high lifetimes and low carrier mobilities that are capable of reducing surface or junction recombination by reducing the flow of carriers into this loss-pathway.