Back in the day, I did some work in the area of dye-sensitised solar cells, and more recently on organic solar cells. So I like to keep an eye on how things are going. Since now and then I hear negative comments about renewable energy - "we're never going to get X% of our energy from it" - I thought I'd post this graph here as it shows that real progress continues to be made and we should be a bit more optimistic (click image for original):
It should also be noted that it may not make much sense to compare efficiencies of different cell types as the price/efficiency ratio may be much different (e.g. semiconductor-grade silicon crystals are more expensive than an organic polymer). Also, efficiency isn't even the full story, e.g. silicon solar cells are top of the pile, but if we all switched to this technology tomorrow, there would likely be a net increase in electricity usage (i.e. when you consider the energy cost of making them). Similarly any technology that relies on rare elements is not going to save the world (but may make a lot of money for some mobile phone makers).
Anyhoo, I like to look at this graph every so often to remind myself that things are improving, and that in the long run, we're going to make it. Or at least as far as efficient solar cells can take us.
For more info on the graph, check out the NREL's photovoltaic page.