The Staebler–Wronski effect was first described in 1977. It refers to the unexplained drop-off in the efficiency of silicon-based photovoltaic cells - as used in solar panels etc - after a few hundred hours of light exposure (So-called 'Light Soaking').
The efficiency can drop by as much as 30%. This has far-reaching implications for the production and large-scale use of silicon solar panels etc.
There are a few suggested explanations of the effect, but none has yet been formally adopted.
See: Wikipedia
Notes:
The effect only applies to amorphous silicon cells (as compared to nano-crystalline types)
It can be repeatedly reversed by heating the cells - this mechanism is also unexplained.
Importance Rating