Currently being sold today! I know SunPower corp. commercial panels are about 20%.
Thats pretty much it for commercially available panels. The only ones more efficient are the one's they put on the Mars landers. They are triple layer panels that catch different spectrum's of light, much more efficient. Millions of dollars more.
New rollable panels are becoming available that don't have to be permanently attached to a surface and that are more resistant to damage than conventional panels. Right now they are also even less efficient than photo cells at about 0%, but the cost factor may make up the difference. Solar thermal is more efficient than photovoltaic cells. They focus sunlight with mirrors onto a pipe containing a liquid which is then heated to it's boiling point. That turns a turbine to produce power or is harnessed in some other way. The panels are cheap compared to photo cells, don't require perfect manufacture or exotic materials, no expensive maintenance and they are much easier to repair. I don't know of any available to individuals yet but there are at least two companies building generating plants in Nevada with this technology. SunPower is about 22% efficient but even at that rate it will take decades to pay for itself and the panels are unlikely to last that long at anything near optimal efficiency. The cost is just too high at present but solar thermal may provide a good alternative until photovoltaic cells mature, if you can cobble together a system. The final link is about a claimed 80% efficient solar cell that even generates some power at night. It's based on nano-technology, like many promising energy developments in batteries, electrolysis and ethanol production. If it works as advertised it should revolutionize solar tech. Eventually.
About 20-22 percent is about right for panels affordable by most consumers. Even then, you still only get enough power to light a couple of lights for $400-$500, and only works when the sun is shining.
There are a lot of solar panels available today's. The problem of the efficiency is the proccess of converting, these cells use the photons of the sun to react with the ( Si compound inside the cell) due to the little thing that cell's are only a fraction of these energy is converted, remember that solar cells are paper thin and the really problem of these cells are the voltage not the amps. A single 3x6 inches solar cells can produce 3.6 amps but only 0.5 volts.
