High Volume Photovoltaic Cell Costs depend on production technique. Large cost reductions are possible.

A cost breakdown and suggestion for reducing costs.

M. Robert Showalter

A manageable task is one in which the expected results can be easily identified; success, failure, or completion of the task can be easily ascertained; the time to complete the task can be easily estimated; and the resource requirements of the task can be easily determined.

from "A Professional's Guide to Systems Analysis", Martin E. Modell, 2nd. Ed. McGraw Hill, 1996.

The task of removing energy as a fundamental constraint on human welfare isn’t manageable without much more specification. But sub-tasks that could contribute to that objective are manageable tasks.

Estimating the high volume mass production cost of a basic Si photocell on the basis of clear assumptions – and how that cost depends on component costs – is a manageable task.

Here is the basic structure of a generic silicon photovoltaic cell

Suppose we assume that 20% efficiency, now available in the best low volume production silicon photocells, is achievable in high volume production. Let “high volume” be defined as a billion square meters per unit time (day, month, year).

(To match fossil fuel energy with PV would take about 100 billion square meters of 20% efficient PV material – about 20,000 gigawatts of capacity. At a billion square meters/year, that would take a century to produce – at a billion square meters/month – 8.3 years – at a billion square meters/week – 2 years. )

It would be a manageable task for glass and automotive engineers to define the cost of assembling layers A, B, F, and an additional glass sealing layer below F at high volume. With ordinary high quality production engineering, that cost, for a 2 mm thickness assembly, would probably be around 1-2 $/square meter ( .5 - 1 cents/watt for a 20% efficient photocell. )

It would be a manageable task for glass, automotive, wire, and textile engineers to design a contact grid layer C. The contact grid might include very small diameter tungsten wire ( ~ 1 micron or less ) adapted for melting into the silicon as the primary contact, with the tungsten wires contacting a larger collection wire grid. Total losses (shadow losses + grid ohmic losses + contact ohmic losses) under 2% should be possible for a high volume cost under .5 $/square meter (under .25 cents/watt for a 20% efficient photocell. )

If the N-type and P-type silicon layers D and E are to be adapted for high volume production assembly with layers A, B, C, F – layers D and E must be in the form of FOILS – about the thickness of light aluminum foil – perhaps 10-15 microns thick. The group 14 element just above Silicon, Carbon is available in foil form. The group 5 element Aluminum, atomic # 13, is available in high volume in foil form. It SHOULD be a manageable task to produce layers D and E in foil form, with the physical properties required for assembly and electrical function.

Summarizing assembly costs in high volume –

Layers A, B, C, F - 1-1.5 $/meter squared - .5-.75 cents/watt.

PLUS the cost of purifying, doping, and shaping the silicon in layers D and E.

The key to reducing photocell costs in very high volume mass production is learning to produce layers D and E in sheet or foil form, at a low enough cost.

If D and E are a total of 20 microns thick, a cost of 1 cent/watt corresponds to $45/kg for the silicon foils forming layers D and E . ( Semiconductor-grade polycrystalline silicon costs about $45/kg today and metallurgical silicon about $2/kg. )

The assumption that high volume production of photocells can be done for under 5 cents/ watt mostly hinges on whether or not silicon sheet or foil of the required properties can be made for less than 40-50 times the cost of a similar quantity of aluminum foil of the same thickness.

The assumption that high volume production of photocells can be done for under 2cents/ watt mostly hinges on whether or not silicon sheet or foil of the required properties can be made for less than 5-10 times the cost of a similar quantity of aluminum foil of the same thickness.

It is plainly physically possible, at some price, to make the silicon sheet or foil required, and to bond layers D and E together. The silicon sheet would have to consist of very large monocrystalline areas. The more pure the silicon ( or silicon doped with B or P ) is – the more feasible forming these sheets would be.

There are probably many possible approaches to producing these silicon foils or sheets within physical laws. ( The energy costs of doing so is theoretically small. The silicon in the sheets could be melted more than 5000 times for a cost less than 1 cents/photocell watt at 20 cents/kilowatt-hr. The theoretical energy cost of making the sheets is much less than .1% of that energy cost. )

I think that it is reasonable to estimate that high volume photocell production costs could be less than $10.00/meter squared – less than 5 cents/watt. At that low price, photovoltaic energy would compete strongly with fossil fuel energy on a wholesale basis.