Advantages and Disadvantages of Organic Devices
The process of fabrication of organic solar cells is relatively easy and cheap, since they do not require very high temperatures to be produced, as for the crystalline silicon in inorganic cells. Organic solar cells are made using wet solution processes or dry thermal evaporation of the organic constituents, what makes it possible for them to be deposited on large areas and on different types of substrates, because of the absence of dangling bonds in the structure. They are mechanically flexible.
Organic semiconductors present a high absorption coefficient which is a measure of the amount of electrons that can be excited from the valence to the conduction band. This results from the narrow electronic bandwidth in these organic materials, what causes the effect of absorption to happen in a limited wavelength range of the solar spectrum. Due to the low dielectric constant of organic materials, the electron diffusion lengths are small when compared to inorganic structures.
The introduction of the bulk-heterojunction concept for the photoactive layer resulted in the increase of the energy conversion efficiency. However, in this junction, the absorbed photons do not release the carriers directly, but rather create excitons. Excitons are electron-hole pairs that are not effectively separated from each other. In other to separate the pair it is necessary to have an energy that is higher than the binding energy between electron and hole. The recombination of the excitons that are not split reduces the efficiency of the organic solar cell.
The charge carrier mobility in organic materials is relatively small, and they may be regarded almost as insulators. This situation takes place because they do not have a three-dimensional crystal lattice and also present local disorders and impurities, causing their energy bands to behave differently from the bands of inorganic materials junctions. Other effects that contributes to a small charge carrier mobility are: (i) the weak forces of interaction (van der Waals forces) present in the organic materials that result in small overlap of electronic wavefunctions and (ii) low intrinsic (thermally excited) charge carrier concentration. Regarding the problem of the lattice disorder, the nanoscopic order can be optimized by the preparation conditions.
Other major concerns while producing organic solar cells are the long-term stability and the control of nanomorphology of the organic materials. The stability may be affected by photodegradation of the active materials, but encapsulation techniques can keep the cell from the presence of oxidizing agents.