If opvdm does not fulfill your modeling needs, Juan Sanchez maintains a nice list of device modeling tools over at TCADcentral. He also publishes a nice looking 2D drift diffusion solver at www.devsim.org
Slivaco also do an organic solar cell simulator, although it looks like it costs money.
The people at TU Delft also do a simulator which is pretty comparable to opvdm in terms of featuers. I've not personaly used it though.