p or n-type wafer for silicon solar cells?

If you pay attention to the wafer polarity of silicon-based solar cells on the market, you will notice that they are mostly p-type.  The most common argument I have heard in classes and textbooks is that electrons have higher mobility than holes, so for the same minority carrier lifetime the electrons will diffuse farther, and thereby increasing collection efficiency.  A couple of renegade companies--Sanyo and SunPower--however, use n-type wafers.  Those two happen to manufacture the highest efficiency single junction silicon solar cells, so surely they are not out of their minds.  What's the real deal here?  (Note that this discussion is meant to be relevant to high-efficiency single crystal silicon solar cells.)

The main reasons for this are historical and date back 40 years ago when silicon solar cells were targeted for space applications.  Early irradiation studies at Bell Laboratories demonstrated that p-type cells were more robust than n-type cells.  Although the n-type wafers had a higher minority carrier lifetime in the beginning, the p-type wafer had a superior end-of-life performance.  Therefore the space industry adopted p-type wafers, and the terrestrial market followed suit.  However, as solar cells made for terrestrial use does not experience such high-energy radiation, the n-type material should be superior.

The mobility argument is simply flawed because it assumes that the minority carrier lifetimes are the same.  It turns out that given the processing conditions necessary to make commercial silicon wafers, hole lifetimes are higher than electron lifetimes.  Combined with the fact that the minority carrier diffusion length easily exceeds the thickness of the solar cell and that higher mobility contributes to lower open-circuit voltage (due to higher dark current), the n-type wafer is therefore superior overall.  In fact, the photocurrent is dominated more by anti-reflection control, light trapping and surface passivation more so than minority carrier mobility.

n-type wafers do not cost significantly more than p-type wafers, so for an application where efficiency matters it is worth it.  Boron, a p-type dopant, has a low segregation coefficient, so it is easier to make uniform and lightly-doped p-type boules.  The high segregation of phosphorus leads to variations in doping during crystal growth.

The greatest concern with n-type absorbers for silicon cells is actually the diffusion step for forming the p+ emitter.  Boron diffusion is more difficult than phosphorus diffusion.

For an in-depth study by folks from SunPower Corp. and the University of New South Wales, see
J.E. Cotter, et al., IEEE Transactions on Electron Devices, Vol. 53, No. 8, August 2006.

1 comment:

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