Electron tomography of ZnO/P3HT solar cells. (a) TEM image of a cross section of a ZnO/P3HT solar cell. (b) Reconstructed volumes of ZnO/P3HT layers. Three samples were analysed, with thicknesses of 57, 100, and 167 nm going from left to right. P3HT is transparent, while ZnO is yellow. (c) Reconstructed volume of a cross section of a ZnO/P3HT active layer in a complete device; (d) an image demonstrating ZnO domains connected to top but not flowing through to the bottom (red arrow) and an entirely isolated ZnO domain (green arrow). Reprinted with permission from Macmillan Publishers Ltd: Nature Materials [71], Copyright 2009.