Krampus Wish List - Photovoltaics

Krampus Gift List - Photovoltaics

This is part 2 of a (hopefully) 3 part series in response to
http://thearchdruidreport.blogspot.com/2013/01/a-wish-list-for-krampus.html

I cover photovoltaics (PV) in this article, in respect to a "post-industrial" world where fossil fuels are depleted and humans must use sustainable sources of energy.  PV - the direct conversion of sunlight to electricity - offers a sustainable supply of electricity, provided one can produce the PV cells and connect them into useful modules.

Terminology

• volt - the unit of electrical potential or "push".  A small single battery is usually about 1.5 volts, barely enough to make a tiny spark.
• amp - short for ampere (A), the unit of electrical current, or quantity of flowing electrons.
• watt - the usual unit of power, that is the rate of energy flow.  One definition is the power of 1 amp flowing through an electrical potential difference of 1 volt.  Roughly, 100 watts is the power given off as heat by a 100 watt incandescent lightbulb, which at 120 volts feed would draw just under 1 amp of current.  My big toaster is 900 watts (W), or 7.5 A at 120 v.
• watt hour (Wh) - unit of energy, the product of rate of flow times time of flow.
• AC. - alternating current, electrical current that reverses polarity, most at 60 (in the U.S.) or 50 (in Europe) times per second.   The standard wall outlet, lighting current.  The alternation allows the voltage to be raised or lowered by inexpensive means of transformers.
• DC - direct current, electrical current that only flows in one direction, like batteries or PV modules.  Most electronics devices use D.C.
• watts peak - Wp - the output of a solar cell/panel under the standard test conditions of light intensity, spectrum and temperature.  The light intensity is defined so that at noon in a temperate latitude, the power of the sunlight is 1000 watts/square meter exactly facing the sun.
• cell - the smallest unit of PV, one photodiode, it produces less than one volt, current depends on area and efficiency.
• strings - used for either cells or modules electrically joined together.
• module - often "panel", one or more cell strings encapsulated, often with a frame.  The smallest installable unit of PV.
• array - one or more modules, in one or more module strings, hooked together to provide DC current when the sun is shining.  May be hooked directly to batteries or a DC load, but most often to an inverter and/or battery charger.
• inverter - electronic device to convert DC current into AC current.

I will use k for kilo- (thousand), M for Mega- (million), and G for Giga- (billion 10^9)

State of PV in the World

Many people are surprised by how big the PV industry's impact is these days.  In 2012 the U.S. installed 11% of the world's newly installed PV, 3,313 MWp, so worldwide that was roughly 30 GWp installed that year. [24]   3.3 GW is about 3 large conventional power plants, though the capacity factor of PV is only about 20%, due to the variability of sunlight, so the output will be less.  But the point should be noted, PV is starting to make real impact in a big way.  At the first of this October, Germany hit 59% renewables on a cool, sunny, windy day, with PV providing a substantial portion, peaking at 20.5 GW. [25]  That's a big amount of power in anybody's book.  In Feb. 2013, it's estimated that cumulative worldwide PV passed the 100 GWp mark. [27]

100 Wp is a small "standard" panel for crystalline silicon, the smallest one in the picture of these Kyocera modules is 140Wp.  It is 59.1 inches (1,501 mm) long x 26.3 inches (668 mm) wide, at max power output gives 7.9 amps at 17.7 volts.  It's module area is 1.003 square meter, so 140 Wp means it's 14% efficient at the module level.  The biggest one to the left is 320 Wp.

A U.S. house uses about 30 kWh per day rough average, though that can be reduced considerably.  So with about 6 hours of full sun/day, a 5 kW (effective) array would supply enough electricity for daily use.  I say "effective" due to things like orientation of module, shading, etc.  And either net metering would need to be used to trade electricity with the grid, or a bunch of expensive batteries used to store energy for nighttime.  If one reduced usage by half, the PV system - modules, mounting, inverter, wiring - would accordingly be half.   Using the 14% efficient modules above, we'd need 35.7 square meters (385 square feet) for the 5 kW system.  Given the average new U.S. house is over 2000 square feet, we'd have plenty of room to put PV modules on the roof, alongside a solar hot water array.

Small (< 10 kW) PV systems in the U.S. are now costing $5.30 per Wp on average, a significant decline from the $12/Wp in 1998.  In Germany, small systems are installed for $2.60/Wp. [26]

So, PV is here, is able to provide adequate amounts of electricity for pretty much any task. Intermittency is solvable with storage, though at increased cost.  Costs are coming down, and conventional utility rates are rising.  Hawaii is already at retail grid parity, as are certain commercial customers in California subject to peak rates.  But there's a long way to go to Terawatt levels of production that would replace most conventional generation.  Can we get there in time with PV and other renewables?

Possible Uses in a "post-industrial" World

I start with some possible uses of PV to give one an idea of just how useful even small amounts of electricity can be.  First is lighting.  Imagine hunting or gathering, or farming, or traveling for trade, or seeking aid (perhaps for a medical/animal emergency) or going to someone's aid, and it turns night.  Imagine the wind is blowing.  Imagine trying to stumble through forest or field, or even along the rough roads of the future, without a light, or a crude torch that keeps going out in the wind.  Did you remember to carry a fragile oil lamp even though you were sure you would be home by dark?  Would an oil lantern be as convenient as a headlamp when farming after dark?  Could you make out that animal in the dark with a flame based lantern as opposed to a flashlight beam?  If you were attending to a injured person after dark, would a candle let you see well enough?

In our modern world, where a light is a switch away, and even most streets and many highways are well lit, it's hard to really comprehend what life without electric lights is about.
The third world has plenty of experience, relying on kerosene lamps with their pollution, fire danger and expense; and is welcoming PV powered lighting. [1]  With LEDs, one can get 8 hours of a small light with just a few "watts peak" (Wp - the output of a solar cell/panel under the standard test conditions of light intensity, spectrum and temperature) of PV.  Many of you may have seen the solar garden lights - something a bit brighter would be plenty useful.

Kerosene lanterns have been in use since the 1850s, and were the first demand source for crude oil.  Besides the health impact, [3], they use more than 1 million barrels/day of kerosene, more than 1% of world oil consumption, and that will not be possible much longer as peak oil hits.

A side note of sorts, using efficient LEDs, a group has developed a gravity powered light. [2]  This has the advantage over PV of needing no batteries, which will need replacing over the years.  It's only practical for stationary applications, and doesn't seem to charge mobile phones..., but the idea has legs.  As someone who has tried windup/crank/squeeze flashlights, I think the gravity light is a great advance.

The next use of electricity to consider is radio.  This can be a simple, receive-only radio that has been in use since the 1920s for news and entertainment.  If people are dispersed to the countryside as farmers, they may not be able to make it to town to get a newspaper, even if there were newspapers (and they could afford them).  The weather, crop prices, and local developments that affect them (say a road closure due to bridge washout, some traders arriving in town, an epidemic of some kind, etc.) can be very valuable in avoiding excess losses and wasted time.

These simple radios could be powered (as many were) by simple wind turbines charging batteries, but there are some parts of the world that have no wind for days at a time, whereas the sun comes up every day.  These kinds of radios need only a few tubes or transistors to function very well, and can be quite cheap.  Tube radios would need a couple of watts, but transistor radios will work on even less power. The other end is the transmitter station, which also needs electricity, though more of it.  A few hundred watts on up for a tube station,
50 to 100 watts or more for transistors, for a simple AM station with a 10 or 20 mile reach.

Next up in complexity would be two way radios, like CB radios and walkie-talkies.  Tube designs would have limited applications and require fairly large PV arrays to power them, but transistor walkie-talkies and CB radios could be charged on 10-30 Wp of PV.

Refrigeration would come next on my doomstead wish-list.  While canning, root cellars, drying and the like work for a great many foods, refrigeration and freezing work for pretty much any food item, including fresh made batches of things like soups, beans, cooked grains, cheese, etc.  Much time can be saved by cooking a large batch of these sorts of foods, and simple re-heating (or not) at mealtime.  Note that climate change means most areas will be getting warmer, and so food will spoil faster.   Another use is for medical purposes, keeping medicines (including herbal preparations) cool, and providing cool for fevered patients.  An interesting book that deals in this is The Frozen Water Trade, which details the distribution of New England ice by sailing ships as far as Calcutta India in the mid 1800s, and the life saving use of ice for tropical fevers.  Some of the solar refrigerators will work off PV arrays as small as 100 Wp, given sufficient insolation. [4]

I might mention that solar thermal refrigeration is also a possibility, using heat from the sun to drive an absorption refrigeration cycle, as opposed to a vapor compression cycle. [5]

Closely following refrigeration might be water pumping, both from wells or from surface water.  While it may be romantic to think of getting a bucket or two of water from a manual well pump, how to get it to your roof for your solar powered hot water heater?  Or pump hundreds of gallons to irrigate some crops?

Once water is pumped, it might be nice to sanitize it, since we're running out of antibiotics that still work to fight infections.  A small air pump and ozone generator will kill most things, and/or a UV light will stifle further growth of living organisms including viruses.  It would be nice to have a disinfectant residue like hydrogen peroxide or chlorine.  Both of these can be generated via electrochemical processes, at fairly small scale, but the peroxide generators require fairly rare metal catalysts and somewhat exotic membranes, and the chlorine/bleach generators are rather fussy, so maybe this isn't a household thing.  But if you knew about such, and had lost a child to dysentery, you might want to get such a unit and the means to power it.

Fans are another PV power-able item it might be handy to have, especially as climate weirding increases.  Small fans can draw just a few 10s of watts.  Full out air condition might be pushing it, but it certainly is possible, especially with efficient ground source heat pumps, though the PV array required gets into the kW range.

Once one gets into the kW range, then electric cookery is possible, as are electric motor based tools.  Electric cooking can be done with slow cookers (though solar typically works fine if you've got the location/sunlight), resistance heaters (toaster, conventional electric stoves), induction cooktops (will need transistors as well), and even microwaves (the cavity magnetron is a surprisingly simple device).

And once one gets in the 10 kW range for a PV array, things like induction forges become possible, for a low carbon, low impact metal industry.  Once one gets to the 100kW or MW range of PV arrays, then one can do things like ... make PV panels.

Approaches to PV

I will mainly discuss crystalline silicon based PV, but first want to give a bit of background on other approaches, most of which are less likely to be around in the future.

One hears (in certain circles) a great deal about organic PV, and how one will be able to just paint your house with some special organic coating and voila!  Well, that is wishful thinking.  For one thing, the photovoltaic effect will only give a voltage up to the "band gap" of the material, and with visible light, that maxes out about 1 volt, typically .6 volt or less.  The looses due to resistance would eat up all the power.  Secondly, all organic PV (dyes or "small molecules") are sensitive to oxygen and water, especially in sunlight.  So for any lasting use, they must be carefully sealed, but even then device lifetimes are on the order of 1000 hours.  Thirdly, especially for dye cells, but also for "small molecule" organic semiconductors, the chemistry is considerably more complex that what I've outline for silicon.

Amorphous silicon (a-Si) is derived straight from silane gas, and its adherents tout the efficient use of that chemical in laying down a thin film (silane can be made from tri-chloro-silane too).  But, a lot is wasted in the deposition process, and the resultant efficiency is low (6%), the long term stability of the a-Si is low and it too has to be carefully sealed.  One finds a lot of it in consumer items, like "solar powered" calculators, but most of them don't spend much time in the sun.  At one time the a-Si guys had a lot of hope, but in the last few years most of the companies producing this have gone bankrupt - unable to compete with plain old boring crystalline wafer silicon.

Copper Indium Gallium Selenide (CIGS) is another thin-film technology, infamous as the choice of Solyndra, the bankrupt PV company.  Other CIGS companies have survived for now, but the consensus in the industry is that outside a few niche markets (flexible, lightweight PV) it just can't compete for big power applications.   CIGS is a very moisture sensitive material, and indium is fairly rare.  While champion efficiencies are about the 20% range, real world modules are closer to 12%.  If one did stumble upon an indium mine, and had access to good polymers for encapsulation, then CIGS might be worth a look.  The inputs are soda-lime glass, molybdenum, copper, indium, gallium, selenium, cadmium, sulfur, zinc oxide or indium tin oxide, some kind of anti-reflective coating like MgF2, nickel or a similar metal, and aluminum for front contact.  Then a good polymer encapsulant and another sheet of glass.  One does a lot of sputtering, so argon gas and vacuum pumps will be needed.  Copper Zinc Tin Sulfide (CZTS) is a similar material, but at a lessor state of development.  CZTS does have the advantage of not using the rare metal indium.

So that leaves cadmium telluride (CdTe) as the sole competitor to wafered silicon PV.   Even this is questionable as the cost of silicon keeps coming down, but for now it has the (barely) cheapest module costs.  CdTe uses both a toxic (cadmium) and a rare (tellurium) element, so there are questions, but frankly somewhat overblown on the toxicity issue.  The tellurium supply issue is open, but worrisome.  Efficiencies are around 12%, comparable to the lowest crystalline silicon on the market.  Both CdTe and CIGS have several manufacturing options, which could be simpler at small scale (assuming raw material availability).   The low efficiency means more glass area is required, and the high moisture sensitivity means glass is a must on the backside as well.  Also, thin films are made in monolithic modules, which means one bad cell requires junking the whole module.  This would also apply in the field.  A glass-glass monolithic module is essentially impossible to fix, whereas even a glass-glass module made of wafers could be drilled from the back to isolate bad cells and jumper around them.  If times are really tough, fixing what one has may be the only option to doing without. 

Making a Silicon PV Cell from a Wafer

I describe a typical process of the standard silicon solar cell.  It is made on a p-type wafer that is typically doped with boron (or could be doped with gallium).  This won't make the highest efficiency, but is the easiest process, accounting for the vast majority (90%) of solar panels in the world today.

First I want to say that PV is not made in "chip fabs" that make integrated circuits for computers, phones and the like.  Aside from university research projects and pricey multi-junction compound semiconductor cells for outer space, nobody in the PV world uses photolithography - all patterning is done with screen printing (the vast majority), ink jets, lasers and the like.  The cleanliness requirements for modern IC processing are literally orders of magnitude more strict than for making PV cells.  One could make silicon cells in one's garage starting from raw wafers if one had a ventilation hood, a hotplate, some suitable processing containers, and an oven that reached 1000 deg. C. (1,800 F).

Step 1 is cleaning/saw damage etch.  Depending on how greasy/dirty the incoming wafers are, one might have to pre-clean with solvents.  Ethanol, acetone, pretty much anything will do.  The solvents could be reclaimed with a solar still.  The main goal though is to remove the saw damage layer, which is about 10 um of cracked silicon contaminated with metals from the sawing process.  This can be combined in a single step using a hot (nearly boiling) and concentrated (around 10 molar) solution of sodium hydroxide.  The sodium hydroxide can be produced as described in part 1.  Sodium hydroxide will slowly etch glass, so it would be greatly preferable to have some plastic container to do this in.  Many plastics would work, polyethylene, polypropylene, ABS, PVC are common ones that one could find as salvage.  Otherwise alumina and a few other ceramics could be used.  I do note that there is a process to make polyethylene out of sugar cane ethanol. [6]

We need some distilled or deionized (DI) water to mix things up and rinse them.  A solar still would work for the distillation.  Put on your goggles, face shield, rubber gloves and apron, then mix the sodium hydroxide (lye) into water.  If your hotplate doesn't do good temperature regulation, use a double-boiler arrangement.  A solar hot water installation could at least pre-heat the heating loop.  Heat a container of sodium hydroxide, load some clean wafers into a basket-like carrier, and dip them in the hot sodium hydroxide.  Set a timer - I recall on the order of 10-20 minutes for this step.  When done, rinse in (DI) water.  The waste contains sodium silicate (water glass), unused lye, silica, and tiny amounts of trace impurities.  You could let it evaporate a bit and reclaim the lye by recystallization, but eventually the silica will mostly precipitate.  You can feed that to the silicon process after washing it, and the impure lye - I dunno, make some soap.

Step 2 is to texture the surface of the wafer for better light trapping, which will increase the efficiency (and decrease use of encapsulation/mounting materials).  For mono-crystalline boules, a lower temperature, more dilute solution of sodium hydroxide is used, along with a surfactant, often isopropyl alcohol (IPA).  Other surfactants could be used, or an old process of fermentation used to produce acetone [7], which can then be hydrogenated into isopropyl alcohol (hydrogen from electrolysis of water, Raney nickel or similar catalyst).  This solution preferentially etches based on the different crystal planes, with the slowest etch rate at the (111) planes.  If starting with (100) silicon wafers, this leaves a surface of random pyramids with a 54.74 degree tip angle. [9]  If you watch the movie in this reference [8] , notice that while the operator is wearing gloves, he is not wearing a "bunny suit" like a modern integrated circuit (IC) fab.  The cleanliness of a PV cell line is much, much less stringent than an IC fab.   High efficiency cells chemically or mechanically polish the back side, either at this step or later.  This makes the back side more efficient, since a larger surface area increases a loss mechanism known as surface recombination.  Basically repeat step 1 with a lower concentration of sodium hydroxide at lower temperature, and add some isopropanol. A cover or reflux condenser will cut your losses of IPA.

Multicrystalline silicon cannot be well textured this way, since the crystals are at random orientations.  Typically a mixture of hydrofluoric and nitric acids is used.  The texture isn't the best, but it's better than doing nothing.

Step 3 is to form the emitter.  This is done by diffusing an n-type dopant, phosphorus, into the front of the cell.  There are two main methods, POCL (phosphorus oxy chloride) or  phosphoric acid.  POCL, chemical formula POCl3, is a volatile liquid that is introduced into a closed furnace via a stream of inert gas, along with oxygen, at around 700 or 800 deg. C. [10]  The furnace will have an electrically heated fused silica tube with the wafers in slots in "boats" of fused silica.   After making a coating of phosphorous "glass" on the wafers, the POCL is turned off, and the temperature raised for a while to "drive in" the dopant.  Typical depth of an emitter made this way is around 1 um.  While this is a batch process and thus less suited to mass production, it is very repeatable and clean.

The other method is to spray a solution of phosphoric acid onto wafers using either an ultrasonic or gas atomization spray. [22]  Some kind of wetting agent, like butanol is used to keep the phosphoric acid from balling up before it dries. [23]  Butanol is a product of the acetone producing fermentation mentioned above.  Then the wafers are typically fed into a belt (or ceramic roller) furnace to "drive in" the dopant at up to 1000 deg. C. [11]  This has the advantage of being a continuous process, but it's not as clean and stable.  At home you can use an electric kiln for this if you don't mind some attack on the heating elements.  No shielding gas is needed, in fact a bit of oxygen from the air is helpful.

Step 4 is the post diffusion PSG (PhosphoSilicate Glass) etch.  The layer of PSG formed during diffusion prevents contact being made with the cell, and particularly for multi crystalline cells, has drawn out a lot of contaminates.  Hydrofluoric acid in fairly dilute concentration is used.  Back to your hotplate and plastic container.  The waste can be neutralized with the waste from step's 1 and 2.

Step 5 is edge isolation, also known as emitter isolation.  In the two main processes for emitter formation described above, the emitter wraps all over the solar cell, electrically shorting the front to the back.  In the old days, a plasma etch was done on a stack of cells, using fluoride containing gases in essentially a big microwave oven.  The gases are bad for the ozone layer and global warming, and the stacking/unstacking damaged a lot of cells.  Then people started using lasers to trim a line around the edge of the wafer on one side or the other.  Now, single-sided wet processes have been developed that combine steps 4 and 5 into one.  The cells are put onto rollers that keeps only one side wetted by an etch solution.  Typically the back is etched and polished using HF and nitric acid, then the wafers flipped into another line which etches the PSG using plain HF.  [12]

At home I think I'd try a tray with a little stand in it.  Set the wafer backside down on the stand.  Use a syringe/pipette to fill the tray up to the bottom of the wafer.  An L-shaped rod would help wet the whole back and keep the HF/nitric acid mixed/circulating.  A bit of air blowing down on the wafer will keep unwanted etching from the front.  When the back is done (via time), take the wafer out, rinse, then toss in a breaker of dilute HF for a time you've empirically determined is long enough.  Remove and rinse and let your new solar cell (albeit without contacts) dry.

Step 6 is typically the deposition of an Anti-Reflective Coating (ARC).  This days it is mostly silicon nitride (SiNx), deposited with plasma enhanced chemical vapor deposition (PECVD). Besides enhancing the light trapping of the solar cell, the SiNx reduced surface recombination losses by the fixed charge in the SiNx, and also trapping some hydrogen from the deposition process which latches onto dangling atomic bonds of the silicon surface, passivating them. [13]

The feedstocks are silane gas and ammonia.  In part 1 I've already described one means of making silane, which can also be derived from trichlorosilane by transforming some of it to silane and the rest to silicon tetrachloride.   This reaction takes place under fairly mild conditions as long as one has a decent metal halide catalyst.

Since the ARC is very thin, less than a micron, not much ammonia is needed.  It may be possible to collect enough urine or other organic source.  If that is not possible, the Haber process could be used, though the pressures are rather high (several thousand psi/150-250 bar).  All one needs is hydrogen, nitrogen, the equipment to pressurize and heat them, and the iron based catalyst. [14]

The PECVD machine is a closed chamber, a batch of wafers goes on a tray in the bottom.  The chamber is pumped down to a vacuum, then a tiny bit of silane and ammonia are allowed in while pumping continues.  An electrical plasma is started to disassociated the gases, turning them into ions and radicals, which in turn deposit on the top side of the wafers.  The power requirements depend on the frequency, but for a batch of 16 wafers is fairly modest, a kW or two.  Frequencies used vary from 50 kHz to 10's of MHz.  Exhaust gases need to be burned, resulting in nitrogen, water and silica that is scrubbed out in water.

If all this is too much, one can grow a layer of silicon oxide with steam or with added silane with just a hot sealed furnace.  It's not the best, but it would be better than nothing, particularly for passivation.

A better (than silicon oxide) antireflective coating would be to use titanium dioxide, as they did in the early 1980s.  One starts with titanium ores (oxides), then reduces them with carbon and chlorine.  The resultant titanium tetrachloride is purified by distillation (it boils at 136 C, 278 F) in the absence of water.  Then it's mixed with isopropyl alcohol to get titanium isopropoxide.  The hydrochloric acid coproduct is certainly useful.  Then one may form the ARC in several ways: spray pyrolysis, screen print a thickened layer then fire it, or spin coat a sol gel.  The latter could use some of the hydrochloric acid as a catalyst. [15]

Step 7 is to "metallize the cell" - put on electrical contacts.  Typically this is done with
screen printing of pastes made from silver particles and glass dusts in a vehicle of various organic things.  There are three separate printings on most cells: [17]
        Ag +glass paste = front gridlines and busbars
        Aluminum paste = rear back surface field to increase efficiency [16]
        Ag paste = rear solderable contacts to back.

The screen printing is just what it says it is, a mesh screen with a pattern that blocks the paste from unwanted areas, and a squeegee that forces paste through the openings, just like making a screen printed T-shirt.  Between putting down each layer, the paste must be dried.

For any surface an ARC was put on (some processes can do it one-sided), the pastes will need the glass particles.  The glass fuses during the firing and dissolves the ARC, allowing the silver or aluminum to make contact with the silicon.   Note that Hall/Heroult introduced their aluminum reduction processes in 1888/1889.

If using TiO2 ARC, one probably waits until after metallization.

One could also do mask defined sputtering or evaporation to put down metal on a cell.

At home, for real simple, one could just paint on some paste.  If one didn't have some nice paste with the glass frit, then make some pure silver particles (grinding or silver nitrate) and use that in some alcohol/etc. as paste before any ARC, then fuse in the furnace at around 950 C.  A reducing atmosphere would be nice here, some hydrogen mixed with mostly nitrogen.  Then I might let in a bit of steam to make a high temperature oxide for surface passivation.  These gas steps are optional.

The back surface field is optional, but nice to have for higher efficiency.  An alternative would be to evaporate or sputter aluminum metal onto the back if one had access to such (a chamber, vacuum pump, argon/power supply/aluminum target or heater/aluminum).  The lack of the back surface field will cost you a percent or two in absolute efficiency.

Step 8 is firing the cells at 800-900 C for a minute or so, which fuses the glass and metal particles and creates contact with the cell. [18]

There, you're done with the cell manufacturing.

Cell Testing

Cells are then tested.  These days with artificial light on automated machines.  They could be testing during the daytime against a known reference cell.  Based on the results, they are then sorted into groups with matching power output.  A string of cells will only produce as much current as its worst cell, so it's a waste to build a string of mis-matched cells. [19]

Module Assembly

From here on out, things are fairly low-tech, or at least they can be.  While most of these steps are automated today, it was only a few years ago that it was all done by hand.  The first thing is to solder tinned copper tabs to the good cells. [20]  These will go from the front of one cell to the back of the next.  An ordinary electric soldering iron and plain old solder work just fine. 

Then a string strings of cells is "laid up" in a line, and the tabs from one soldered to the next in line.  A cell gives somewhere between .6 and .7 volts, so one needs about 4 cells for a minimally useful module.  The full sized modules have strings of about 9 cells, and join 4 strings, giving roughly 20 volts output at no load, dropping some under load.  This is a convenient voltage to charge 12 volt batteries with.

After testing the string, some number of strings are laid up, typically on top of the front glass and a layer of encapsulant material, and soldered together by copper ribbons interlayered with more encapsulant material.  Then another sheet of encapsulant goes over the whole thing, then either a plastic backsheet or another piece of glass, then the whole things goes to a laminator.  In the laminator, there is a membrane above the module assembly, and the whole thing is inside a metal box that is now sealed.  A vacuum pump pumps the air out of both the top and bottom compartments, the assembly is heated so the encapsulant melts, then air is let back in the top, so everything is squished together.

After the lamination, one will then attach either a junction box and/or some wire leads to the ends of the ribbons left outside the encapsulant.  Typically these boxes are made of molded plastic, but I'm thinking they might just be dispensed with.  Wire insulation is another issue (is every thing in the modern world made of plastic?), it has to deal with years of outdoor exposure.

There - now you have a solar module, ready to be tested.

The polymer based encapsulants (and backsheet) are the rub.  The most common today is Ethylene-Vinyl Acetate (EVA).  Currently in widespread use for a great many plastic products, it is now made from petroleum.  Ethylene can be generated from ethanol from fermentation.  The vinyl acetate is made one of two ways: ethylene + acetic acid + oxygen, or acetic acid + acetylene.  Acetic acid is what vinegar is made of, by fermentation of ethanol by acetic acid bacteria.  So we have to make a lot of booze to make PV I guess.

Many early solar modules used polydimethylsiloxane (PDMS)  based silicones, but those were/are expensive compared to EVA.  Synthesizing PDMS would involve getting some methanol, likely via the destructive distillation of wood - we need the charcoal anyhow.  The methanol gets mixed with hydrochloric acid to make methyl chloride, which gets reacted with some of the metallurgical grade silicon from our electric arc furnace to make dimethyldichlorosilane (among other things).  After some separation via distillation, we can then figure out how we want to cure it.

A site specific analysis of the economic factors involved would be necessary before making a decision on which way to go.  Both EVA (at least the well-made stuff) and silicone encapsulated modules have lasted 25+ years.

If either polymer (or alternatives) simply was not easily available, then you could make a module by mounting the cells on an insulated metal sheet pan, and bolting that to a sheet of glass.  It need not be completely sealed to take care of thermal expansion of the gases inside, though a breather plug to keep the bugs and dust out would be good.  Outgassing of interior components should be avoided, a fibrous mineral insulation could be used, or blown glass insulators strung together on metal wire.  There are many ways to skin the PV cat.

Another issue is whether to use a polymer backsheet or another sheet of glass.  Unfortunately, the only polymers that hang around long enough for a reasonable module life (25 - 30 years) are made of fluorinated plastics, and are also produced on fairly large scale on rather sophisticated calendering/laminating machines.   I'm thinking another sheet of glass, that we don't have to bother making low iron, will be the way to go.  Rather than building a float glass plant and a backsheet plant, just use the float glass plant more.

Discussion

Looking in back through the steps I don't find any complete show stoppers.  Sulphuric acid to make the other acids seems easy enough.  Sodium hydroxide is easy as long as we have electricity and salt water.  The scary one was looking for plastic containers to do the wet processing in, and we'll either salvage that or do ethanol based polyethylene.  The SiNx ARC is the hardest part I think, for two reasons.  One is the RF plasma generator, though there are successful systems using both microwaves and lower frequency (50 kHz), at a few hundred watts, and that ought to be fairly easy.  The other is the ammonia, which is a big undertaking to get the combination of pressure and temperature.

Concerns about the lack of sophisticated semiconductor fab are misplaced, the first PV cells were made by guys dressed in wool suits.  If you look at picture #4 in this reference, you see the early diffusion ovens, even for transistors, were basically overgrown kitchen ovens.  The women operators aren't even wearing hair nets. [21]

Encapsulants will be the big issue, which will affect any PV technology.  If we assume a 12% efficient (poor for now, but way better than nothing) module, down due to the more crude nature of things, and a .5 mm layer of EVA, for 1 MWp we need 8,334 square meters x 2 sides x .0005 meter thick = roughly 8.4 tonnes of EVA assuming no waste.  Thats are fair bit of booze to come up with without industrial farming.

An open issue are the UV protectants for the EVA, and the curing catalysts for both the EVA and silicones.  Some of these chemical are fairly exotic.  And there's a lot of empirical information that is often proprietary to the companies that now produce the polymers, which means that in a more rapid/disruptive collapse that re-inventing the needed subset of the plastics industry will not go so smoothly.

CIGS, CZTS and CdTe might be worth looking at - it would take some real detailed study to see what would really be sustainable.  But for now the market is voting crystalline silicon.

Conclusion

Again, I can't really conclude without knowing more about the environment the proposed operation would find itself in.  The current existence of many 10's of GWp/year production capacity says things are possible, but the difficulties of reduced transportation and less access to petrochemicals are open issues.  Turning silicon wafers into PV is not a one-person operation if one had to make everything by oneself, but gets rather involved with a lot of parts and chemicals.  But neither is it a priori impossible to assemble a team of dozens of workers at small but usable scale.

==========

references

[1] http://www.cnn.com/2012/01/10/tech/innovation/solar-powered-led-lamps/

[2] http://deciwatt.org/

[3] http://light.lbl.gov/pubs/tr/Lumina-TR10-health-impacts.pdf

more info on low-carbon off-grid light for the developing world:

http://light.lbl.gov/pubs.html

[4] http://sundanzer.com

[5] http://www.treehugger.com/clean-technology/new-solar-refrigerator-prototype-from-chile.html

[6] http://opensourceecology.org/wiki/Polyethylene_from_Ethanol

http://www.braskem.com.br/site.aspx/plastic-green

[7]  http://en.wikipedia.org/wiki/Acetone-butanol-ethanol_fermentation

[8] http://www.pveducation.org/pvcdrom/manufacturing/texturing

[9] http://memslibrary.com/guest-articles/47-silicon-etching/33-anisotropic-wet-etching-of-silicon-with-alkaline-etchants.html

[10] http://sun.anu.edu.au/files/ltbfp_PhosDiffn.pdf

[11] http://www.pveducation.org/pvcdrom/manufacturing/emitter-diffusion

[12] http://www.ise.fraunhofer.de/de/veroeffentlichungen/konferenzbeitraege/2008/23th-european-photovoltaic-solar-energy-conference-valencia-spain-2008/single-side-etching-key-technology-for-industrial-high-efficiency-processing

http://www.rena.com/fileadmin/img/Produkte/200_Solartechnik/220_Inline/222_InOx/RENA_DB_InOxSide_20110829_final2.pdf

[13] http://pveducation.org/pvcdrom/design/anti-reflection-coatings

http://pveducation.org/pvcdrom/manufacturing/anti-reflection-coatings

[14] en.wikipedia.org/wiki/Haber_process

[15] http://www.journalamme.org/papers_vol52_1/5211.pdf

[16] http://pveducation.org/pvcdrom/design/surface-recombination

[17] http://pveducation.org/pvcdrom/manufacturing/screen-print-front

[18] http://pveducation.org/pvcdrom/manufacturing/firing

[19] http://pveducation.org/pvcdrom/modules/mismatch-for-cells-connected-in-series

[20] http://pveducation.org/pvcdrom/manufacturing/module

[21] http://www.computerhistory.org/semiconductor/timeline/1954-Diffusion.html

[22] http://www.sono-tek.com/chemcoat-silicon-doping

[23] http://iopscience.iop.org/0022-3727/40/9/008

[24] http://www.seia.org/research-resources/us-solar-market-insight-2012-year-review

[25] http://www.greentechmedia.com/articles/read/Germany-Hits-59-Renewable-Peak-Grid-Does-Not-Explode

[26] Tracking the Sun is now put out yearly by the Lawrence Berkeley national lab
http://emp.lbl.gov/sites/all/files/lbnl-6350e.pdf

[27] http://www.renewableenergyworld.com/rea/news/article/2013/02/100-gw-of-solar-pv-now-installed-in-the-world-today

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edits
20140118 - some typos about removing the PSG, fixed factual error that tube radios require 10's of watts due to existence proof of Russian thermo-electric generator powered radio at:
http://www.douglas-self.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm#rl
90 V * 0.012 A = 1.08 W,  + 1.5 V * 0.25 A = 0.375 W = 1.455 W
Added 100 GWp of PV cumulative install.