Solar panel

By mass a crystalline silicon solar panel is about three quarters glass, a tenth aluminium in the frame, and the remainder polymer encapsulant, silicon, and the metals that carry the current off it.

The silicon is the thinnest part and the whole point. A cell is a wafer around 150 micrometres thick — thinner than a business card, and steadily getting thinner because silicon is the expensive ingredient — doped to make a junction, textured to trap light, and coated with silicon nitride, which is what makes panels blue.

The layer that matters commercially is the one nobody sees: a screen-printed silver paste forms the fine grid of lines across the cell face. Silver because it is the best conductor there is and because it must be printed and fired at temperature without oxidising. Photovoltaics now consume on the order of a sixth of world silver production, which was not true a decade ago.

The rest of the module is protection. Low-iron glass to let light through, an EVA or polyolefin encapsulant to seal the cells, a polymer backsheet, an aluminium frame, and a junction box — assembled to survive twenty-five years outdoors, which is what the warranty is actually about.

The chain is unusually long for something that ends up looking so simple, and every step is somewhere different.

Quartz sand is reduced with carbon in an arc furnace to metallurgical silicon. That is purified through trichlorosilane to polysilicon — the Siemens process, and the most energy-intensive step by a distance. Polysilicon is melted and pulled into a single crystal by the Czochralski method, the boule is squared off and sliced into wafers with a diamond wire saw, and the sawing loses a substantial fraction of the silicon as kerf.

Wafers are then textured, doped by diffusion to form the junction, coated with antireflective silicon nitride, screen-printed with silver and aluminium, and fired. Cells are strung together with tabbing ribbon, laminated between glass and backsheet with the encapsulant, framed and tested.

Essentially all of this now happens in China, and much of the polysilicon step in Xinjiang, where coal-fired electricity makes it economic.

The panel is the clearest example there is of a manufactured object whose price fell by roughly ninety-nine per cent in thirty years, and the reasons are materials reasons as much as manufacturing ones.

Silicon per watt fell because wafers got thinner, saws got finer and cell efficiency rose from around fifteen per cent to over twenty-two. Silver per cell has been cut repeatedly by printing narrower lines, and copper plating has been pursued for decades as a replacement without ever quite displacing it.

What has not fallen is the concentration. Polysilicon, wafers, cells and modules are each dominated by a small number of Chinese producers, and the manufacturing advantage is real — scale, integration and cheap power — rather than a subsidy story alone.

The material to watch is silver. A sixth of world production going into an industry that intends to grow several times over is a genuine constraint, and it is why every manufacturer has a copper metallisation programme and why none has fully switched.

Energy payback is between one and two years depending overwhelmingly on where the polysilicon was made, against a service life of twenty-five to thirty. Manufactured on hydroelectricity the figure is nearer one; on coal, nearer two. That single variable moves the answer more than any design choice.

End of life is the unsolved part and it is arriving. A panel installed in 2010 will be retired around 2035, and the volumes then are very large. Recycling recovers the aluminium frame and the glass easily and profitably, and the silicon, silver and copper — the parts worth having — poorly, because they are laminated behind an encapsulant that was designed never to come apart. Most panel recycling today is glass and frame recovery with the laminate discarded.

The cadmium telluride thin film alternative reverses several of these: far less semiconductor, far less energy to make, a lower efficiency, and a producer-run take-back scheme built precisely because the cadmium makes disposal a regulated question. It holds a steady minority share.