It should be realized that there has been a great deal of progress in the science and technology of organic solar cells. In particular, the measurements, standardization of methods for testing efficiencies, materials, structure-function relationships, as well as the details of the photo-physics and long-term stability have thoroughly been investigated in a number of promising organic solar materials. In considering the basic solar cell device structures, we can see that the earliest organic solar cells were based on single thermally evaporated molecular organic layers sandwiched between two metal electrodes of different work functions. In a typical experiment with a bilayer device with an organic material, the first process is a photon absorption in the donor, which is followed by an electron being excited from the ground state to the first excited state of the molecule's electronic structure. The use of an inverted geometry enables the use of such metals as silver as the hole-collecting electrode.