Light from the sun contains a range of wavelengths ranging from UV to IR.
Typical PV (photovoltaic) materials in common use work best at visible light wavelengths and "waste" energy at either extreme. Less commonly used and usually more expensive materials are available that can be optimised to work best at either UV or IR wavelengths. By combining cells that work most efficiently at various wavelengths it is possible to make cells that produce more energy per area from typical sunlight.
Light reaching the earth's surface is reduced in amplitude overall by passing through the earth's atmosphere. Far UC / UV-C / short wavelength UV is essentially completely absorbed, UV-B A (longer wavelength) is very significantly absorbed and UV-A (longer again wavelength) is somewhat absorbed.
Triple junction cells are optimised to work across the full solar spectrum. The abikity to deal with UV-C is wasted for terrestial PV panels and triple junction cells are usually targeted at space applications. Accordingly, no cost is spared in their manufacture and optimisation and they are both not vastly more useful than double junction cells with a somewhat enhanced UV and IR coverage, and much more expensive.
Air effects on solar insolation are standardised for test purposes - the Air Mass 1.5 = AM1.5 conditions usually being quoted. This is the light that results when solar insolation passes through the atmosphere such thata 1.5 x as much mass of air is passed through as is the case at true noon when the sun falls vertically. ie at about 45 degrees. (See figure at end of this post)
Wikipedia Am1.5 et al
If mounting area is not limited and weight is not a crucial factor then using cells which optimise energy output long term per total "net present value" $ spent is usually best. If space is constrained (yacht, backpack, limited roof area etc) or if windage matters (hurricanes, roofrack, ...) or weight (roof loading, back pack) or all of these are utterly crucial (across Australia solar challenge ...) then highest possible efficiency per area may matter. But in most cases just buying the reasonably most efficient commodity PV cells is good enough.
As well as efficiency uou may care about lifetime. Crystalline silicon PV laminated in glass usually comes with a 20+ year still running at > 80% original output guarantee or promise, and actual lifetimes of over 30 years are achieved by competently built units. Early CIGS and CdTe cells had limited lifetimes (as little as a few years) and while vastly longer lifetimes are now being claimed by some manufacturers you may wish to do due diligence on the probability of their claims being true in any given case.
Monocrystalline silicon laminated on low iron glass can provide whole panel efficiencies of over 15% with cell efficiencies of over 17% commonly advertised. While some people may advertise 20%+ efficiencies for some technologies in some cases you are generally into expert care and buyer beware situations.
Looking on ebay for monocrystalline silicon and working out offered efficiencies (17%+) and cost should give you as good a deal as most available.
A quick look on ebay shows this offering as typical of an apparently OK deal. They say:
US made 125x125 Monocrystalline Solar Cells 2.8 Watt Mono 5x5 Photovoltaic wafer
Monocrystalline Photovoltaic Solar Cells Made in USA, by BP Solar, Frederick, Maryland
All cells, completely new, mostly in factory packs of 50 or 25 cells per pack. The price listed is for one 5"x5" solar cell not the whole pack. You can purchase any qty that you need. (10, 50, 100, 1000, we got you covered)
125mm x 125mm
Efficiency upto 17.75 %
Wikipedia multijunction PV
Triple junction amorphous on stainless steel.
BUT still only 13% total from here
40% in 2010 !!! from here - buying them may be another matter.
Atmospheric effects on optical transmission can be modelled as if the atmosphere is concentrated in approximately the lower 9 km.(From above Wikipedia ref)