A look at the top candidates for creating the age-old cells.

It has been close to 30 years, but it is only now, as the solar cell industry strives for increased efficiencies, that selective emitter is being taken seriously. Indeed, one reviewer of the past Intersolar North America Expo noted that just about every crystalline-silicon cell manufacturer worth its salt is now devoting R&D resources to its development.¹

It’s easy to see why. The energy-converting heart of the majority of silicon-based solar cell is its p/n junction. This is normally formed in the early stages of cell manufacture by firing the wafer in a phosphorus-rich atmosphere, thereby diffusing a uniform, planar layer of phosphorus a few hundred nanometers into the upper zone of the wafer. Here, photons of sunlight release electrons that migrate through the silicon to the cell’s front face, where they are captured by the grid of silver conductor fingers printed on the cell’s top side. Once captured, they flow around the circuit to the aluminium contact on the cell’s reverse side to rejoin their electron-poor, “holey” atoms, and in so doing, they create the cell’s electrical current.

Maximum efficiency relies on everything working optimally – on the photons generating sufficient electron/hole pairs, and on these migrating to the right places and being collected properly. Herein lies the rub. The very material that is instrumental in giving the p/n junction its functionality also forms a significant barrier to light: The top part of the cell where the phosphorus is most concentrated can in fact “waste” a massive 30% of incident light in the blue part of the spectrum.

Selective emitter mitigates these losses by limiting higher concentrations of phosphorus to areas directly under the silver collection grid. Here, the phosphorus can do no harm, as this area of the cell is in shadow, and at the same time, it improves the contact between the silicon and grid, facilitating electron migration. The area between the grid, on the other hand, contains relatively low levels of phosphorus, optimizing the cell’s blue response and increasing efficiencies.

That’s the why; the how is a little more complex. As is inevitable for a technology that has been kicking around for so long, numerous techniques have been developed in labs over the years, all promising to create the definitive selective emitter solar cell. Time will tell which technologies win out, but in the meantime, we can pare the likely contenders to six or seven main categories.

In most of these, the wafer is first doped with far lower phosphorus concentrations than the current industry standard. Extra dopant is subsequently added in the areas directly in contact with the silver conductor grid. This can be achieved in several ways.

Perhaps easiest and cheapest, if not most effective, is to use hybrid pastes. These are standard metallization pastes containing added dopants, enabling both to be printed at the same time. The phosphorus then diffuses into the silicon as the paste is fired.

Another way of adding extra dopant is to screen-print it onto the wafer in a grid pattern that is perfectly aligned to the silver collection grid that will subsequently be printed over it.

An alternative to printing is selective diffusion, by which a second doping step is performed prior to the metallization step, but with the areas between the silver grid fingers masked off so that only the grid area is doubly doped. In a slight variation on this theme, semipermeable masks are being developed that enable the wafer to be doped to different concentrations in a single step.

Laser doping, developed by the University of Stuttgart’s Institute for Physical Electronics, laser-writes the phospho-silicate glass (PSG) layer that forms on the wafer surface during the standard diffusion process, driving more phosphorus into the silicon under the area where the silver grid will be printed. The PSG layer is then removed, as in standard processes.

In the etchback process, the wafer is doped at high concentrations, after which the wafer surface in the areas between the fingers is etched away, taking with it much of the phosphorus it contains. This can be achieved using printed etching pastes, or by using etch masks and a wet chemical etching process. A variation of this replaces the standard doping process with doping paste, which is printed across the entire wafer and then etched off
where appropriate.

One of the most complex but interesting alternatives is the buried contact process, whereby deep trenches are laser- or saw-cut through several passivation, doping and emitter layers. These trenches are further doped and then metallized, creating a collector grid that extends deep into the wafer and is fine enough to reduce the shadowing effect, and boost cell efficiencies, enormously.

All these approaches come with pros and cons, but are interesting enough to attract the solar industry’s attention and resources. To be serious contenders, however, they must be easy to use, increase production efficiencies, and their added process steps should not compromise the already delicate structure of the solar cell, either in production or in the field. To achieve this, the environments in which they are to be introduced must be capable of delivering repeatable alignment accuracy and, given the fragility of silicon solar cells, extremely careful handling at all stages during the process, all without compromising throughput rates.

References
1. Tom Cheyney, “Intersolar North America 2010 Redux: PV Multiplicity, Surging CIGS, Single-Crystal Silicon, and More,” PV-Tech.org Daily News, July 28, 2010.

Tom Falcon is a senior process development specialist at DEK Solar (dek.com); tfalcon@dek.com.

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