Photovoltaic panels can have 20 or 25 year underwritten warranties with a guaranteed remaining efficiency of 80 % of the new panel. That means, that photovoltaic panels seem to degrade somehow.
Degradation of PV cells has been studied for some time. The dominant factor in such degradation seems to be UV exposure. This seems to occur on two fronts.
1) Encapsulant. The PV cells themselves are contained in a layer of encapsulant, usually EVA (ethyl vinyl acetate), which undergoes slow changes when exposed to UV. First, it can change from transparent to brown, cutting down on light reaching the cells. Second, it can release acetic acid, which corrodes connections. Third, it becomes less elastic and lets water vapor into the cells, which also causes corrosion.
2) Cell structure - PV cells are actually great big diodes, and depend on a fairly precise profile of dopants in the silicon. UV seems to have enough energy to cause slow, small diffusion of the dopants away from their desired concentration profiles, reducing the efficiency of the cells.
To a lesser extent, thermal cycling also causes fatigue in electrical joints, which behaves much like overt corrosion, and also works in tandem with the encapsulant changes listed above.
As to prolonging PV lifetime, protecting them from UV and temperature cycles seem the best measures. Since this means keeping them out of the sunlight, it's clear that this has limited usefulness for working systems.
The encapsulant problem can be reduced by using panels which employ silicone rather than EVA. Unfortunately, silicone is more expensive than EVA.
Not a complete answer - think of this as added notes:
Glass + EVA + Crystalline Silicon + EVA + backsheet:
20-25 year lifetimes are typically attained from good quality panels from reputable manufacturers in real-world conditions with panels permanently mounted in their service position.
Real and available panels are getting 20++ years in those conditions.
I have an old rather degraded but still running BP Solar panel that is > 30 years old.
All EVAs are not created equal and I have seen EVA advertised on Alibaba for "In China use only".
A manufacturer I dealt with in China would use only German EVA and identified it as a major advantage in achieving longer lifetimes than his competitors.
The maker of the best small PV panels that I have found uses Chinese EVA.
Original PV6100 introductory paper - the product has quietly vanished from the market but the requirements and reasons are still valid. 2008 - Australian National University & Dow Corning joint paper.
NREL - types of encapsulant materials and differences
NREL 2008 - accelerated UV test methods for PV encapsulants
ebay ad with useful information
Silicone rubbers are useful but "tricky" for PV use.
Dow Corning introduced their magic PV6100 SR a few years back and it quietly vanished from the market over about 2 years - suggesting that it had unexpected priblems.
Dow Corning Sylgard 184 is available but very niche.
PV Encapsulants do not work as most people think.
Their main task is NOT to keep water out but
To ensure that the corrodable material is exposed to water vapour and not liquid water. As wv has ~ 1000 x lower water density than liquid water the corrosion rates are proportionally slower all else being equal.
Minimising dissolved water % in the SR helps address point 1.
Creating voidless contact with encapsulated materials addresses point 1.
Also, materials should be "low modulus' aka stretchy and rubber like to allow thermal and other stresses to be minimised. SRs can be substantially better than EVA in this respect.
Not form corrosive degradation products.
Not form transparancy degradation products.
Not degrade :-) - or, degrade as little as possible in high UV environments. The Si-Si bonds in the main chains have bond energies somewhat above the quanta energy in most terrestial UV - unlike eg epoxy resins C-C bonds which are able to be broken by UV quanta energy.
Low cost is always nice.
Water levels in a glass+ EVA panel equalise to ambient levels in a few months.
SR's are typically 10x MORE water permeable than EVA. What is needed is low DISSOLVED water %, no voids and low modulus (stretchy). Suitably designed SR's CAN be better than EVA in these respects.
Available solar encapsulant SRs include
GE RTV 615
Dow Sylgard 184 & 164
OptiTec 7020
QSIL 216 & 220
I have only seen results for Sylgard 184 which is well spoken of. Others may be as good but I have no information. QSil is said to use Dow silicone products in its formulations
Their "shelf-life", from what I remember, is indefinite. Being bombarded with light is the degrading factor. If they were getting sunlight for 24 hours a day the lifetime would decrease faster that the 20-25 year life. A lot is marketing, like with LED fixture lamps, they say "lasts 20 years" but the fine print says "if used 3 hours a day".
This may be an interesting question for the chemistry stack. As far as I can recall, a photon hits the [Si] and the [Si] loses an electron (giving you power). The atom that was struck by the photon becomes [Si+] and (I believe) [Si+] doesn't have the same photovoltaic properties.
Here is a pretty good link for the diode structure and operation of a cell: http://www.pbs.org/wgbh/nova/solar/insi-nf.html
