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Written by Tom Falcon   
Monday, 31 October 2011 15:19



Combining wafer technologies to yield satisfying results.

We are to the point in the photovoltaics market where standard wafer types alone just won’t cut it from an efficiency point of view. Fortunately, there are proven “recipes” for improving performance, and by combining some novel technologies, cell efficiency can be significantly enhanced. In fact, this is precisely the way the industry is moving, so we’ll devote this month’s column to a review of various wafer types and different techniques/technologies that can be applied to each to improve performance.

As I see it, there are four different types of wafers:

  • P-type: This wafer type accounts for roughly 85% of today’s silicon solar cells and is a structure where the bulk of the silicon is boron-doped, with a thin flash of phosphorus on the top. This category can be subdivided again into mono-crystalline and poly-crystalline.
  • N-type: With N-type wafers, the structure is exactly the same as the P-type, but the bulk silicon is doped with phosphorus (N-doped), and the emitter layer is doped with boron (P-doped). So, in effect, the polarity of the cell is reversed. Evidence suggests this is advantageous, as it reduces cell degradation over time (as compared to standard P-type wafers).
  • Metal wrap through (MWT): MWT wafers are effectively P-type wafers with approximately 16 uniformly drilled holes, which act as vias and conduct current from the front side of the cell through to the back side. (See Solar Technologies, May and July 2011.)
  • Emitter wrap through (EWT): EWT wafers are another form of back contact wafer technology (as is MWT), but with EWT, there are in the range of 50,000 small laser-drilled holes doped simultaneously with the front side of the cell, enabling conductivity from front to back.

One can then use the above wafer structures as the base and add to them one/some/all of the below technologies to further enhance cell performance. I’ve broken down the various technology add-ons as follows:

  • Print-on-print (PoP): PoP is a technique we have discussed at length in previous columns. In a nutshell, the technology utilizes two-stage screen printing to achieve taller, thinner conductor fingers that improve cell efficiency by reducing shadowing, while enabling greater current collection.
  • Dual print: Dual print, a process developed by the Energy Research Centre of the Netherlands, also utilizes double printing similar to PoP, but busbars are printed first with a screen, and then the fingers are printed with a metal stencil.
  • Selective emitter: This technology aims to mitigate efficiency losses by limiting the higher concentrations of phosphorus to those areas of the wafer directly below the collector grid, thereby facilitating electron migration. There are multiple techniques for adding the extra dopant in the areas beneath the conductor grid, and many of these were reviewed in an earlier column (“Which Way for Selective Emitter,” January 2011). These include the use of hybrid pastes, screen printing, selective diffusion, laser doping, etchback and ion implantation.
  • Rear side passivation: While nearly all wafers have a passivated emitter, it is most often on the front facing side of the wafer. The reason for passivating the rear side of the wafer is twofold: First, it acts as an optical reflector, so instead of light just passing straight through the wafer, some of it bounces back and goes through the wafer a second time, enabling twice the opportunity for interaction with an atom to generate current; second, rear side passivation reduces recombination. (This is where electrons recombine with atoms locally instead of carrying current around the circuit.)

Now we get to the fun part: the various “solar stew” recipes. By taking certain wafer types and adding various technologies, cell performance can be measurably improved. For example, either of the different print techniques (PoP or dual print) added to a standard wafer may yield in the range of 0.2% to 0.3% efficiency. If done well, selective emitter may increase conversion efficiency as much as 0.4%, and rear side passivation has potentially the biggest gain of between 0.3% and 0.7%. So, all told, several used in combination could easily offer close to a full percentage point increase in efficiency, a huge number in the solar world. And, surprisingly, with three of the four wafer types, all of the efficiency-enhancing techniques could be applied either individually or in any combination. Table 1 illustrates how, by using different technology combinations with various wafer types, solar cell efficiency can be improved.

Tom Falcon is a senior process development specialist at DEK Solar (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column runs bimonthly.

Last Updated on Tuesday, 01 November 2011 12:59
 

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