Today I came across a graph I prepared two years ago: the number of papers published per year in scientific journals within the field of organic photovoltaics. 
I just updated it using Web Of Science, up to year 2007.
In case you want to reproduce the graph, I used the topic
“organic photovoltaic cell” or “organic photovoltaics” or “organic solar cell*”
for organic photovoltaics and related phrases, and
“bulk heterojunction solar cell*” or “bulk-heterojunction solar cell*” or “polymer photovoltaic” or “polymer fullerene photovoltaic” or “polymer solar cell*” or “polymer fullerene solar cell*” or “polymer-fullerene solar cell*” or “polymer-fullerene solar cell*”
Web of Science can also combine search sets in the history, so that publications matching both sets are not counted twice; the result is shown as the curve “both” in the graph. Probably, by a more appropriate choice of search terms, even some more papers can be found. For instance, I should have included small molecules.
The result is not strictly growing exponentially, but the interest still is increasing continuously. Let’s hope that the commercial interest will have similar growth rates soon;-)
Notes on Disordered Matter 