Recently, I added a section to the sidebar of Solar Panel sources at prices of $4 or less per watt. A recent search turned up a few more sources under the $4/watt figure, one at $3/watt though the $3/watt panels were CIGS technology which, although less expensive, is both less efficient and shorter lived than silicon panels, particularly mono-crystalline solar panels.
When you look for Solar Panels, the most common types you will find are mono-crystalline, poly-crystalline silicon, amorphous silicon, and CIGS (Copper Indium Gallium Selenide) (Cu(In,Ga)Se2) [Thanks to the anonymous commenter who pointed out my error.]
Of these mono-crystalline panels are the most expensive; there are even more expensive multi-junction solar cells and solar panels manufactured but they are so expensive that they are generally reserved for space applications where the power-to-weight ratio far outweighs the additional expense given the huge cost of getting something in orbit.
Mono-crystalline solar panels are generally the most expensive solar panel designed for terrestrial applications, however, they are the most efficient single junction panel and they have the longest lifespan. Mono-crystalline solar silicon panels will generally provide efficiencies of 15-19% and lose less than 10% of their power production capability in thirty years. You should look for panels with a glass, not plastic face because plastic will discolor over this time frame and reduce efficiency.
Poly-crystalline silicon solar panels are less expensive than mono-crystalline panels but also have lower efficiencies, generally in the range of 12-15%, and poly-crystalline silicon solar panels will usually lose about 10% of their capacity in the first couple of years. Thereafter they will generally remain stable for decades.
Amorphous silicon solar panels use a thin film of silicon deposited on a substrate such as steel or glass. Those that use steel as a substrate are flexible. They typically have low efficiencies ranging from about 6% to 10%. Although they cost less to produce, market demand is presently much greater than supply because their flexible nature allows them to be used in products where other types of solar cells can not be used, such as roll-up portable battery chargers, and at present their retail cost does not reflect the lower cost of production and in fact tends to be higher than other silicon panels. Because many different substrates and disposition methods are used to produce these cells, little is known about their long term stability and it is likely to vary widely with the substrate and manufacturing process used.
CIGS technology solar panels are very inexpensive to produce, but they have lower efficiencies and deteriorate faster than other solar panel technologies. Although they are often advertised as having a lifetime of twenty years, they really haven’t been around long enough to know what their long term performance will be like. Although much less expensive to produce than silicon panels, the present market saturation conditions tends to keep their retail prices at around 75% of those of polycrystalline solar panels. However, as production is ramped up they have the potential for producing power at well under $1/watt and at that rate, a shorter lifetime and lower efficiency may well be an acceptable trade-off for much lower costs in many applications.